Inductively Coupled Plasma Etching in ICI- and IBr-Based

Chemistries: Part I. GaAs, GaSb and AlGaAs

Y. B. Hahn(')*, D. C. Hays ('I, H. Cho"), K. B. Jung (l), E. S. Lambers('), C. R. Abernathy('), S. J. Pearton('), W. S . Hobsod2), and R. J. S h ~ l ' ~ )

(1) Department of Materials Science and Engineering, University of Florida, Gainesville,

(2) Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974 (3) Sandia National Laboratories, Albuquerque, NM 87185.

FL 3261 1.

ABSTRACT

High density plasma etching of GaAs, GaSb and AlGaAs was performed in ICVAr and

TBr/Ar chemistries using an Inductively Coupled Plasma (ICP) source. GaSb and AlGaAs

showed maxima in their etch rates for both plasma chemistries as a function of

interhalogen percentage, while GaAs showed increased etch rates with plasma

composition in both chemistries. Etch rates of all materials increased substantially with

increasing rf chuck power, but rapidly decreased with chamber pressure. Selectivities >

10 for GaAs and GaSb over AlGaAs were obtained in both chemistries. The etched

surfaces of GaAs showed smooth morphology, which were somewhat better with ICVAr

than with IBr/Ar discharge. Auger Electron Spectroscopy analysis revealed equi-rate of

removal of group I11 and V components or the corresponding etch products, maintaining

the stoichiometry of the etched surface.

* Present address: Department of Chemical Engineering and Technology, Chonbuk

National University, Chonju 56 1-756, Korea.

1

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that i ts use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

DISCLAIMER

Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

INTRODUCTION

In the fabrication of high frequency transistors as well as optoelectronic devices,

it is critically important to accurately control the pattern size, with minimal damage

created during the patterning process. The 111-V compounds such as GaAs, GaSb and

AlGaAs are used for high electron mobility transistors (HEMTs), heterojunction bipolar

transistors (HBTs), lasers and light-emitting diodes (LEDs).(') The trend toward

decreasing feature size has become an important issue and to this end various types of dry

etching techniques have been under development. Reactive ion beam etching (RIBE)

allows the ion energy and ion flux to be controlled inde~endently,(~-~) but the ion energies

are too high for electronic device fabrication.

High density plasma etching techniques have been reported to provide high etch

rates for GaAs and related materials using Cl2-based or BCL-based plasma~.(~' '~) Most of

the previous work has been focused on Electron Cyclotron Resonance (ECR) sources, in

terms of etch rate and surface morphology or etch profiles, but little work has been

reported on using Inductively Coupled Plasmas (ICP). The latter are the prefened

embodiment of the high density plasma concept, with excellent uniformity and

controllability.

Interhalogens such as IC1 and IBr have been reported to be readily dissociated

under ECR conditions, producing high concentrations of reactive specie^.('^.'^) Etch rates

of 1.2 pdmin for GaAs and 0.7 pmlmin for GaSb were reported in ECR ICVAr

plasmas.(I6) However, no work has been done on the ICP etching of 111-V compounds

2

with IC1- and IBr-based plasma chemistries. These chemistries appear very attractive for

high-rate etching of 111-V compounds, for applications such as through-wafer vias.

In this work, the influence of interhalogen etch gases (IC1 and IBr) in ICP etching

of GaAs, GaSb and AlGaAs was carried out for various plasma parameters. The effects

of plasma composition, rf chuck power, and ICP source on the etch rates, dc bias and ion

fluxes, and morphology have been investigated. The ICP ICVAr and IBr/Ar discharges

resulted in high etch rates for the typical 111-V semiconductors, but there is no clear

advantage in terms of etch rates and surface chemistry for either chemistry.

EXPERIMENTAL,

The samples used for etching in this work are: semi-insulating undoped (100)

GaAs and undoped (100) GaSb substrates grown by the Czochralski process, and

nominally undoped (p - 10l6 cmJ) Alo.zsGao.nAs grown by either Metal Organic

Molecular Beam Epitaxy (MOMBE)('*) or Metal Organic Chemical Vapor Deposition

(MOCVD)('~) at 550 - 650 OC on semi-insulating GaAs substrates.

The samples were patterned with Apiezon wax and etched in a Plasma-Therm ICP

790 system. The system consists of etch gas feed lines, a 2 MHz ICP source (ISOOW),

and a He backside-cooled rf (13.56 MHz) powered sample chuck. The rf chuck power

was varied between 50 and 350 W, and ICP source between 300 and 1000 W. The

chamber pressure was varied from 5 to 20 mTorr, while the total flow rate of the gas

mixture was 15 sccm. Etch rates were calculated from stylus profilometry measurements

of the etched samples with measuring error of approximately 2 5%. The morphology and

3

near-surface chemistries of the eched samples were examined by atomic force

microscopy (AFM) operating in tapping mode with Si tip, and Auger Electron

Spectroscopy (AES), respectively.

RESULTS AND DISCUSSION

Figure 1 shows the effect of plasma composition on etch rates of GaAs, GaSb,

and AlGaAs in IBr/Ar and ICY& discharges at 5 mTorr, 750 W source power and 250 W

rf chuck power. The etch rate of GaSb increased up to 33.3 % of interhalogen gas by flow

in both ICY& (Fig. 1, top) and IBr/Ar (middle) discharges and decreased thereafter.

AlGaAs showed maximum etch rates at 33.3 % IC1 and 66.7 'YO IBr, respectively. The

attainable maximum etch rates were similar in both chemistries: 1.75 p d m i n for GaSb

and 400 min for AlGaSb. The etch rate of GaAs increased with increasing interhalogen

content in both discharges. This result indicates that etching of GaAs in either chemistry

is more attributed to chemical etching by increased concentrations of reactive neutrals

than by ion-assisted sputtering, which is the mechanism for GaSb and AlGaAs.

The dc self-bias voltage increased with increasing etch gas concentrations,

resulting in a decrease in ion flux entering the sheath layer (Fig. 1, bottom). The ion flux

at the sheath edge was calculated using a global self-consistent model developed for the

ICP etching system.(20) The increase in dc biases or decrease in ion flux is attributed to

additional collisional energy losses due to the presence of interhalogens.(21)

Figure 2 shows the effect of ICP source power on etch rates, dc bias voltages, and

ion fluxes at the sheath edge for ICY& (top) and Il3r/Ar discharges (middle). Flow rates

4

of etch gases were held constant at 2 sccm IBr or IC1 and 13 sccm Ar. During these runs

the chamber pressure and the rf chuck power were held constant at 5 mTorr and 250 W,

respectively. Up to 500 W all materials showed gradual increases in etch rates. However,

at higher source powers (> 500 W) the etch rates of GaAs and GaSb increased

substantially: this leads to etch selectivities of > 10 for both GaAs and GaSb over

AlGaAs, which is etched slowly in both mixtures due to the low volatility of the AlI, and

AlBr, products. The increase in etch rate with increasing the source power is due to the

higher concentration of reactive species in the plasma, suggesting a reactant-limited

regime, and to higher ion flux to the substrate surface. Lower dc biases were attributed

mainly to increased ion density at higher ICP powers (Fig. 2, bottom).

The effect of rf chuck power on the etch rates, dc bias, and ion flux at the sheath

edge is shown in Fig. 3. Etch rates for all materials increased in both IC1 (top) and PBr

(middle) discharges as the rf power or the ion-bombarding energy increased. The increase

in etch rate with the chuck power can be attributed to enhanced sputter desorption of etch

products. The dc bias voltage increased monotonically with increasing rf chuck power

from 50 to 350 W, but the ion flux at the sheath edge increased slightly (Fig.3, bottom).

This is because the main role of the chuck power is to increase the ion-bombarding

energy. The effect of the rf power on etch rate (or etch yield) and ion flux at the sheath

edge in the ICP system is described in detail elsewhere.(20)

Figure 4 shows the effect of reactor pressure on etch rate, etch yield (defined as

number of atoms etched per incident ion), dc bias and ion flux in ICVAr plasmas. During

these experiments the source and chuck powers were held constant at 750 W and 250 W,

respectively. The etch rates of all materials decreased with increasing pressure. This is

5

products. Etch yield data are shown in the lower part of the figure. The higher dc voltages

or lower ion fluxes at higher pressures were attributed to increased collisional

recombination which decreased the plasma ion density.

I

Etched surface morphology was examined using AFM for GaAs samples etched

at 750 W ICP power, 250 W rfchuck power and 5 mTorr in 2 sccm ICY13 sccm Ar and 2

sccm lBr/l3 sccm Ar discharges, respectively. The AFM results are shown in Fig. 5 with

the rms roughness. It is seen that ICVAr chemistry (top) shows somewhat better

morphology than ICUAr (top), but both surfaces are fairly similar to unetched controls,

which show rms values of 0.7 - 1.1 nm..

In addition to the surface smoothness, equi-rate removal of group 111 and V

components or their corresponding etch products are very important to guarantee the

stoichiometry of the etched surface. Figure 6 and 7 show the AES surface scans and

depth profiles of GaAs etched in, respectively, ICVAr and IBr/Ar plasmas at 750 W ICP

power, 250 W chuck power and 5 mTorr. There is oxygen present that grows on the

samples in the course of transfer from the ICP chamber to the AES system and also

carbon contamination due to the exposurre to surrounding air. As shown in the depth

profiles of Figs. 6 and 7, the etched surfaces with both interhalogen discharges are

chemically quite clean. It is also seen fi-om the A E S scans that the etched surfaces remain

stoichiometric, indicating equirate of removal of group I11 and V components in both

plasma chemistries.

6

SUMMARY AND DISCUSSION

A parametric study of etching GaAs, GaSb and AlGaAs has been carried out with

ICl/Ar and IBr/Ar chemistries in an Inductively Coupled Plasma discharge. The effects

of plasma composition, ICP source power, rf chuck power and chamber pressure on etch

rate, etch yield, dc-bias voltage and ion flux at the sheath edge were examined. GaSb and

AlGaAs showed maximum etch rates depending on plasma chemistry and interhalogen

percentage, while GaAs etch rates were proportional to the interhalogen content in both

chemistries. Etch rates of all materials in the IC1- and IBr-based discharges decreased

with reactor pressure, but increased substantially with increasing rf chuck power,

indicating that higher bombardment energies are more efficient in enhancing sputter

desorption of etch products. ICl/Ar plasma showed somewhat better morphology of

etched GaAs than IBr/Ar discharge. AES analysis revealed equi-rate of removal of group

III and V components and maintenance of stoichiometry on etched surfaces.

ACKNOWLEDGEMENTS

The work at UF is partially supported by a DOD/MURZ monitored by AFOSR (H.

C. DeLong), contract no. F49620-1-96-0026. Y . B. Hahn gratefully acknowledges the

support of the Korea Research Foundation for Faculty Research Abroad and KOSEF

through the Automation Research Center. Sandia is a multi-program laboratory operated

by Sandia Corporation, a Lockheed-Martin company, for the US Department of Energy

under contract DEAC-94AL-85000.

7

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42,43 1 (1996).

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Electon. Mater., 26,266 (1997)

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13. R. J. Shul, G. B. McClellan, S. A. Casalnuovo, D. J. Roeger, S. J. Pearton, C .

Constantine, C. Barratt, R. F. Karlicek, Jr., C. Tran, and M. Schurmann, Appl. Phys.

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Figure Captions

Figure 1. Effect of plasma composition on etch rates in ICVAr (top) and IBr/Ar (middle)

plasma chemistries, and dc bias and ion flux at the sheath (bottom).

Figure 2. Effect of ICP source power on etch rates in ICVAr (top) and IBr/Ar (middle)

plasma chemistries, and dc bias and ion flux at the sheath (bottom).

Figure 3. Effect of rf chuck power on etch rates in ICVAr (top) and IBr/Ar (middle)

plasma chemistries, and dc bias and ion flux at the sheath (bottom).

Figure 4. Effect of process pressure on etch rates in ICVAr (top) plasma chemistry, and

dc bias and ion flux at the sheath (bottom).

Figure 5. AFM scans for GaAs etched in ICVAr (top) and IBr/Ar (bottom) plasmas.

Figure 6. AES surface scan (top) and depth profile (bottom) of GaAs etched in 2ICV13Ar

plasma at 750 W source power, 250 W rf chuck power and 5 mTorr.

Figure 7. AES surface scan (top) and depth profile (bottom) of GaAs etched in 2IBr/l3Ar

plasma at 750 W source power, 250 W rf chuck power and 5 mTorr.

10

2.00 750 W ICP, 250 W rf, 5 mTorr n

S .I . E E 1

+ AlGaAs w m Omo4 P /-\

- 1 1 ct 5 0

0.02 I I I I

0 20 40 60 80 100 Yo IC1

350 s I w

8 300

250

.I m

200

I I I I I I

0 20 40 60 80 100 % IBr 1.54 n

v) + dc bias (IBr/Ar) r

- -W- dc bias (ICVAr) 9 U . IonFlux(IBr/Ar) E -

0

- 1.52 FO x

- x =I L c

(D -Ct - IonFlux(ICVAr) -

T-

U - m

I I I I 1.50 D

0 20 40 60 80 100 % IBr or IC1

n

8000 E 3 6000

Q) w 2 4000 c

2000

0 5

0

800

600

400

200

f -t- GaAs 250 W rf, 2IC1/13Ar, 5 mTorr + AIGaAs

GaSb /

0 200 400 600 800

0 200 400 600 800

250 W rf, 2IBr/13Ary 5 mTorr

+ GaAs -I)- AlGaAs --&- GaSb k

0 200 400 600 800

+ dc bias (IBr/Ar) -D- dc bias (ICVAr) -U - IonFlux(IBr/Ar) -U- - IonFlux(ICVAr)

0 200 400 600 ICP Power (W)

800

2.0 n T

v) 9 . 1.5 E 0

(D . 1.0 '0 F L

0.5 X 3 - L

0.0 0 -

30000

20000

10000

0

t -0- GaAs + AIGaAs 750 W ICP, 2ICU13Ar

5 mTorr / + GaSb

600 I I

400 200 0 M 0 100 200 300 400

0 100 200 300 400

750 W ICP, 2IBr/13Ar 5 mTorr

20000 - -+- GaAs

GaSb 10000 -

0

+ AlGaAs

0

2000 - I000 -

0 1

I

0 100 200 300 400 . 1.56

1.52

1.48

-@- de bias (IBr/Ar) 400

+ de bias (ICVAr) -0 - Ion Flux(IBr/Ar) --c) IonFlux(ICl/Ar)

n 300 > I U (D

r 0 3 200 F U

X 3 L S 0

- m

.I m g I00

0 0 100 200 300

rf Chuck Power (W) 400

n c E .I

3 8000 U

350

n 300

U

v) m .I

Po o 250

200

12000 750 W ICP, 250 W rf 2ICVl3Ar

-@- GaAs -C- AlGaAs -A- GaSb -

-

400

5 10 15 20

3

S I w

0

5 10 15 20

, 2.5 + GaAs + AlGaAs + GaSb

I

\ I 5 10 15 20

- 2.0

- 1.5

- 1.0

Pressure (mTorr)

1 oonm

2IC1/13Ar RMS Roughness = 0.9nm

loom T

2IBr/l3Ar RMS Roughness = 1.6nm

c

10

9

8

7 3 a 6

w- 5 L W w 4

z 3

2

1

8 280 480 6-m 8Q0 lB00 1288 1480 1600 1808 2000

Kinetic Energy (eV)

18

9

8

- 1: 0 ;-

-- Ga-

0. 0 0.5 L0 1.5 2. 0 2.5 3.0 3.5 4.0

Sputter Time (min)

I

10

9

8

7

a 6 9 8 w- 5 n w w 4

3 z

2

1

0

-?

C

288 488

I

I As Ga

Kinetic Energy (eV)

10

9

8

7 56 $ 5

4 4 E 3

2

1

0 0.0 0.5 1. e 1.5 2. 0 2.5 3.0 3.5 4. e

Sputter Time (min)